1. Field of the Invention
The present invention relates to an electrode having high capacity and good reversibility, and a secondary battery comprising i) a positive electrode comprising an organosulfur compound which is capable of reversible formation of S--S bond upon oxidation and an electroactive compound selected from main group elements, and a current collector containing copper metal ii) a polymer electrolyte having lithium salt; and iii) a negative electrode made of lithium metal, lithium alloy or lithium intercalation compounds.
2. Description of Prior Art
Battery has a wide spectrum of applications as a key component of modern portable electronic devices. Especially, secondary battery is essential to the development of hand-arrying devices such as mobile telecommunication tools and notebook computers. A series of development of nickel-cadmium, nickel-metal hydride, and lithium ion types has provided advantages in reducing the size and weight of secondary battery. However, the rapid advancement of electronic technology and widespread use of portable devices has been continuously demanding the next generation battery which has higher capacity than existing systems.
The secondary battery using organosulfur compound as a positive electrode material has been disclosed in U.S. Pat. No. 4,833,048. In this patent, the sulfur-sulfur bond of organic disulfide compound consisting of positive electrode is cleaved by electrolytic reduction to form organic thiolate and disulfide bond is regenerated by electrolytic oxidation of organic thiolate. Especially, in case of two or more thiolate groups present in a molecule, formation of disulfide bond leads to polymeric form of organic disulfide compound. The redox couple of organic disulfide and organic thiolate accounts for theoretical energy density of 350 to 800 Wh/kg in combination with metal negative electrode. A rechargeable metal-organosulfur battery described in the invention provides practical energy density of 150 Wh/kg which is higher than that of conventional secondary battery.
To increase the practical capacity of organic disulfide electrode, U.S. Pat. No. 5,324,599 suggested the addition of .pi. electron conjugated conductive polymer such as polyaniline to cathodic composition containing organic disulfide. According to the report of same inventors disclosed in Nature, 373, 598(1995), the electron transfer of organic disulfide was catalytically accelerated in the presence of polyaniline. Accordingly, the composite electrode from organic disulfide and polyaniline mixed together in molecular level shows the enhanced energy density in excess of 600 Wh/kg when coupled with lithium metal as negative electrode. However, in order to maintain the high energy density, the cell required charging voltage up to 4.75 V, which is too high to ensure the electrochemical stability of cell components such as polymeric electrolyte and other organic parts. Lower charging potential which is practically required results in the decline of energy density of the cell.
In order to increase the cycle life of organic disulfide electrode, approaches to immobilize organic disulfide have been made since diffusive loss of soluble form of organic disulfide, such as mercaptan or thiolate, eventually results in the deterioration of capacity over the repeated cycle of charge and discharge. Addition of metallic compound such as copper, or silver which have binding affinity to organic disulfide species was disclosed in U.S. Pat. No. 5,665,492. Addition of copper ion to organic disulfide and employment of the resulted complex was described in Eur. Pat. No. 799,264, A2. Improvement of cycle life was also suggested in U.S. Pat. No. 5,516,598, in which metal salt of broad range of multivalent metallic complex of organic disulfide was used. In these disclosures, the role of metal as a coordinating center of sulfur containing ligand were described to improve the cycle life of organic disulfide electrode, but functions of metal such as redox reaction and activation of sulfur containing compound were not suggested. Consequently, above mentioned approaches provide only limited level of energy densities which represents at maximum the sole capability of organic disulfide.
Accordingly, the improvement for enhancing capacity and extended cycle life has yet to be realized to the secondary battery employing organosulfur compound as positive electrode material.